Lightening skin via tyrosinase modification

However altered production of cutaneous melanin may cause considerable aesthetic problems, especially hyperpigmentation conditions such as melasma, postinflammatory hyperpigmentation, freckles, or lentigines (age spots).1 Skin pigmentation is the result of a blend of pigments, predominantly made up of melanin, but carotenoids and haemoglobin also contribute. Melanin production occurs in melanosomes, lysosome-like organelles found within melanocytes.2 There are two different melanin compounds, eumelanin and pheomelanin, which differ in colour, but both have the same biochemical origin. Both melanins are polymers synthesised from the amino acid L-tyrosine, which is first hydroxylated to form dopa, and then oxidised to form dopaquinone. Both reactions are catalysed by the enzyme tyrosinase. At this point the synthesis of the two melanin pigments diverges. If cysteine or glutathione are present, they conjugate with dopaquinone to form benzothiazine precursors of the red/yellow pigment pheomelanin. Alternatively, as cysteine availability diminishes, dopaquinone cyclises into dopachrome where it is catalysed first by TYRP2 and then TYRP1 to monomeric indolic precursors that ultimately form the blackbrown pigment eumelanin.3 A mix of pheoand eumelanin is produced and deposited onto the melanosomal matrix proteins, giving rise to skin pigmentation.1 Tyrosinase is the key regulatory enzyme in melanogenesis and it catalyses the first two steps in the melanin biosynthetic pathway. It is a copper containing glycoprotein which is initially synthesised in the form of a 55kDa protein, and undergoes post-translational modifications before being transferred to the melanosomes as a 75kDa glycosylated protein.4 PhynoRadianceTM is a proprietary plant extract that is rich in a group of compounds known as imino sugars. Imino sugars are monosaccharide analogues in which the ring oxygen is replaced with a nitrogen atom. The simplest forms resemble furanose and pyranose monosaccharides, with the most common naturally occurring ring structures being pyrrolidine, piperidine, indolizidine, and nor-tropane. Six-membered piperidine imino sugars, such as 1-deoxynojirimycin (DNJ), which is a glucose analogue, are the most studied due to their ability to mimic analogous pyranoses in their interactions with carbohydrate-processing enzymes.5 DNJ and other imino sugars have been shown to potently inhibit ?- and ?-glucosidase, enzymes that are responsible for the cleavage of glucose from polysaccharides or glucose substituted molecules.5 ?-glucosidase plays an important role in the maturation of tyrosinase. Tyrosinase is a type-1 membrane glycoprotein and is synthesised in the endoplasmic reticulum (ER) where it acquires the correctly folded conformation for its catalytic activity, before transferring to the Golgi, and finally to the melanosomes. During the maturation and folding process, the nascent or immature tyrosinase undergoes transformations which include N-glycosylation. During this process, glucosidase enzymes trim N-linked glycans off the polypeptide, which allows the interaction of the chaperone molecules calnexin/calreticulin. Optimal folding of tyrosinase is dependent on its association with the chaperone molecules that modulate the folding of the nascent tyrosinase through their interaction with attached N-glycans. Inhibition of glucosidase enzymes therefore does not allow for correct folding of the nascent tyrosinase, resulting in inactive mature tyrosinase.6 This mechanism of action represents a different approach to skin lightening compared with commonly-used skin lighteners that are often direct inhibitors of tyrosinase. Arbutin is a glycosylated hydroquinone and competitively and reversibly binds to the active site of tyrosinase, inhibiting the conversion of tyrosine to L-dopa, and L-dopa to L-dopaquinone.7 Other direct inhibitors of tyrosinase include mequinol, N-acetyl-4-S-cysteaminylphenol (NCAP), kojic acid, azelaic acid, gentisic acid, aloesin, and hydroxystilbenes (e.g. oxyresveratrol). Antioxidants and redox agents such as ascorbic acid derivatives and ?-tocopherol also have a skin lightening effect as they prevent oxidative damage to the skin and inhibit secondary messengers that may stimulate melanogenesis. Redox agents can also interact with copper at the active site of tyrosinase or with O-quinones to impede oxidative polymerisation of melanin intermediates. Magnesium-L-ascorbyl-2- phosphate interferes with melanin synthesis by reducing oxidised dopaquinone and interacting with copper ions at the active site.3 This paper examines the in vitro and in vivo activity of PhynoRadiance (now referred to as ‘the lightening active’) and explores further the different mechanisms of action of the imino sugar rich ingredient.

Materials and methods

Direct vs. indirect inhibition of tyrosinase